This invention relates in general to load limiting struts and, more specifically, to a passive nonlinear support strut particularly useful as supports for spacecraft on an upper stage during first stage launch vehicle ascent.
A variety of different load alleviating supports have been developed for reducing adverse effects of varying loads on structures. Typical of these are automotive-type shock absorbers, various hydraulic cylinders and the like. These systems are effective in lightly loaded, single load direction applications such as automobiles and the like, but are not capable of accommodating very large loads and loads that vary in both compression and tension.
More complex variable struts, such as that described by Belew in U.S. Pat. No. 4,536,114, have been developed for space applications such as truss and beam structures. These systems tend to be complex, have a number of active components and are not always sufficiently reliable for high load launch vehicle applications.
Snap through buckling structures in which bars are caused to buckle between two configurations are effective in alleviating loads that occur sequentially in opposite directions. Such systems are described by Farley et al in NASA Technical Briefs magazine, June 1989 at page 77. This is a very advantageous technique, although it is not capable of effectively absorbing intermittent compressive and tensile loads. Similarly, Belville washer assemblies function as a passive structure capable of alleviating compression loads. However, these assemblies are not fully effective in reacting both compressive and tensile loads and do not provide desirable redundancy and simplicity.
A particularly difficult application for load limiting struts occurs where an upper stage carrying a heavy spacecraft is launched on an initial stage, such as a space shuttle or the like. Initial stage ascent forces can impose forces from the spacecraft onto the upper stage that are beyond the structural capability of the upper stage to react. During first stage lift-off and ascent flight operation, concentrated loads are imposed on the spacecraft to upper stage interface, while after engine cut off tension forces are imposed, all of which must be accommodated by the interface struts. One end of each of the spacecraft and upper stage can be rigidly fastened to the launch vehicle, i.e. to the cargo bay structure of a space shuttle or the like. The other end of each of the upper stage and spacecraft must have some freedom of movement in the vehicle axial direction to accommodate the re-distribution of induced loads. Present strut and load relieving systems as described above have limited load carrying capability, and have insufficient reliability, especially in the case of active load alleviation systems.
Thus, there is a continuing need for improved load alleviating system of improved simplicity and reliability that are capable of reacting large loads in both compression and tension in severe environments, such as space launch vehicle systems.